The present invention relates to a liquid crystal display apparatus having a level conversion circuit in which a signal having a low voltage amplitude is converted to a signal having a high voltage amplitude; and, in particular, the invention relates to a level conversion circuit for use in a clock interface and a data interface of a liquid crystal display apparatus using thin-film transistors (TFT: Thin-Film Transistor).
A level conversion circuit of the type used in a clock interface and a data interface of a liquid crystal display apparatus is described in, for example JP-A 6-216753 and JP-A 6-283979. In the level conversion circuit shown in these publications, a thin-film transistor, such as a multi- crystallization silicon and a metal-oxide semiconductor (MOS: Metal-Oxide Semiconductor) having a mono-crystallization silicon, are employed. In such a level conversion circuit, an input signal having a low voltage amplitude is converted to an output signal having a high voltage amplitude for use in a drive circuit for the liquid crystal display apparatus.
The above-stated input signal has, for example, a voltage amplitude of 5 V or 3.3 V, such as used in a common LSI. Further, the above-stated output signal has, for example, a voltage amplitude of 12 V or 15 V, which corresponds to a power supply voltage of an interior circuit of the level conversion circuit.
As examples of this level conversion circuit, there are a differential input type level conversion circuit, which inputs a mutually reverse phase signal, and a single phase input type level conversion circuit, which inputs an independent signal. The differential input type level conversion circuit is used in a comparatively high speed clock interface, and the single phase input type level conversion circuit is used in a data interface.
FIG. 9 shows an example of the differential input type level conversion circuit described in JP-A 6-216753. This level conversion circuit 800 is constituted by a pair of input transistors 811 and 812, a pair of load transistors 813 and 814, a pair of constant current power supplies 815 and 816, and a pair of level shift transistors 817 and 818.
The respective input transistors 811 and 812 and the respective level shift transistors 817 and 818 are each provided as an N type TFT. The respective load transistors 813 and 814 are each provided as a P type TFT. In the level shift transistors 817 and 818, a drain electrode and a gate electrode are connected to each other and respective source electrodes are connected to input terminals VIN1 and VIN2. Further, to a connection point of the drain electrode and the gate electrode, the constant current power supplies 815 and 816 and the gate electrodes of the input transistors 811 and 812 are connected.
The respective source electrodes of the input transistors 811 and 812 are connected to ground and the respective drain electrodes of the input transistors 811 and 812 are connected to the respective output terminals VOUT1 and VOUT2. The respective drain electrodes of the load transistors 813 and 814 are connected respectively to output terminals VOUT1 and VOUT2. The respective gate electrodes of the load transistors 813 and 814 are connected respectively to the output terminals VOUT2 and VOUT1. The respective source electrodes of the load transistors 813 and 814 are connected to a power supply VDD.
In the level conversion circuit 800 connected in the above-described manner, the signals which are supplied at the input terminals VIN1 and VIN2 have a mutually reverse phase. Herein, the operation state of the level conversion circuit 800 will be explained on the assumption that the voltages which are inputted to the input terminals VIN1 and VIN2 are 3.3 V and 0 V, respectively, the voltage of the power supply VDD is 15 V, and a threshold voltage of the respective N type transistors is 2 V.
Since each of the level shift transistors 817 and 818 operates to increase the voltage level at the input terminals VIN1 and VIN2 with a threshold voltage, the voltages of 5.3 V and 2 V are applied respectively to the gate electrodes of the input transistors 811 and 812. As a result, the input transistor 811 presents a conductive state and the input transistor 812 presents a non-conductive state, respectively, and then the voltage of the output terminal VOUT1 becomes 0 V.
Since this output terminal VOUT1 is connected to the gate electrode of the load transistor 814, the load transistor 814 presents a conductive state and then the voltage of the output terminal VOUT2 becomes 15 V. Further, since the load transistor 814 whose gate electrode is connected to the output terminal VOUT2 becomes a non-conductive state, then the output terminal VOUT1 maintains the voltage of 0 V.
Next, from the above-described state, the operation wherein the voltages of the input terminals VIN1 and VIN2 change respectively to 0 V and 3.3 V will be explained. When the voltages of the input terminals VIN1 and VIN2 change respectively to 0 V and 3.3 V, the input transistor 811 presents the conductive state, but the input transistor 812 presents a non-conductive state, respectively.
At this time, since the load transistor 814, which is connected to the drain electrode of the input transistor 812 becoming the conductive state, presents the conductive state, when the resistances at the conductive states of the input transistor 812 and the load transistor 814 are expressed by RON2 and RON4, the voltage VOUT2 of the output terminal VOUT2 at the time at which the voltage of the input terminal changes is expressed by the following formula 1.
VOUT2=RON2/(RON2+RON4)xe2x96xa1VDDxe2x80x83xe2x80x83(1)
As understood from the above-stated formula 1, the voltage of the output terminal VOUT2 at the time at which the voltage of the input terminal changes is determined by a divided voltage ratio between the resistances RON2 and RON4. With the above stated voltage, the load transistor 813 presents the conductive state and the voltage of the output terminal VOUT1 changes to 15 V. Since the voltage of the output terminal VOUT1 changes to 15 V, the resistance of the load transistor 814 increases, and finally the load transistor 814 presents a non-conductive state. As a result, the voltage of the output terminal VOUT2 becomes 0 V.
Herein, to shorten the time from when the conductive state of the input transistor 812 occurs to the time when the voltage of the output terminal VOUT2 becomes 0 V, it is necessary to make the voltage of the output terminal VOUT2 approach 0 V as soon as possible by making the resistance value of the resistor RON2 small in the formula 1.
On the other hand, in the single phase input type level conversion circuit, one approach is employed using the differential input type level conversion circuit explained above, in which a single signal is inputted to one input terminal and a voltage having xe2x96xa1 of the single phase input amplitude is supplied to the other input terminal; or another approach is employed using the differential input type level conversion circuit explained above, in which a single signal is inputted to one input terminal and the single phase input amplitude is supplied to the other input terminal by reversing the single phase signal.
When the voltage between the drain electrode and the source electrode is constant, the drain current of a TFT or MOS transistor changes in proportion to a square of the effective gate voltage VE, which is a difference between the gate voltage and the threshold voltage Vth. Since the resistance RON, such as RON2 and RON4, under the above-stated conductive state is in inverse proportion to this drain current, the gate voltage increases abruptly in the vicinity of the threshold voltage Vth.
In the case of the above-described level conversion circuit 800, the drive condition of the gate voltages of the input transistors 811 and 812 corresponds to a voltage Vg1 between the gate electrode and the drain electrode of 2 V and a voltage Vg2 between the gate electrode and the drain electrode of 5.3 V.
To obtain the resistance under the conductive state necessary for a voltage Vg2 of 5.3 V, it is necessary to make the size of the input transistor large. When the size of the input transistor is made large, the capacity of the input terminal increases and also the capacity between the gate electrode and the drain electrode increases.
Further, since the input transistors 811 and 812 of the level conversion circuit 800 shown in FIG. 9 constitute a source ground type amplification circuit, the capacity between the gate electrode and the drain electrode is made large equivalently with a magnification of an amplification by the Miller effect. The increase in the equivalent load capacity becomes an obstacle to achieve high speed operation.
Further, the conventional single phase input type level conversion circuit is more complicated in comparison with the differential input type level conversion circuit. This means, for example, that, when an attempt is made to apply the single phase input type level conversion circuit to a digital type liquid crystal display apparatus, which is operated by an image signal in the form of a digital signal, the complexity becomes a large obstacle.
When an image is sent in the form of a digital signal, the data size thereof depends on the number of colors (a number of gray level) in the image signal, the pixel element number of the liquid crystal display apparatus, the frame frequency, and the operation frequency of the level conversion circuit. For example, in a case where the number of colors (number of gray level) requires 8 bits, the pixel element number is 1280 xe2x96xa11024, the frame frequency is 60 Hz, and the operation frequency is 20 MHz, the input data size is about 32 bits.
An object of the present invention is to provide a liquid crystal display apparatus having a level conversion circuit which is able to carry out a high speed operation in the liquid crystal display apparatus.
Another object of the present invention is to provide a liquid crystal display apparatus having a level conversion circuit which can be constituted by a small transistor capacity in the liquid crystal display apparatus.
According to the present invention, the above-stated objects can be attained in a liquid crystal display apparatus, wherein a level conversion circuit is provided for a signal circuit for driving the pixel elements of a display unit and a scanning circuit, the level conversion circuit is constituted of a first transistor and a second transistor in which the respective gate electrodes of the first transistor and the second transistor are connected to a first bias voltage power supply, and a third transistor and a fourth transistor in which respective gate electrodes of the third transistor and the fourth transistor are connected to a second bias voltage power supply and respective drain electrodes of the third transistor and the fourth transistor are connected to a power supply.
The respective drain electrodes of the first transistor and the second transistor are connected to respective drain electrodes of the third transistor and the fourth transistor, a signal having a mutually different polarity and having a low amplitude is inputted to the respective source electrodes of the first transistor and the second transistor, and from the drain electrode of the first transistor and the respective drain electrode of the first transistor and the second transistor, a signal having a mutually different polarity and having a high amplitude is taken out.
In accordance with the present invention, an output voltage is determined in accordance with a resistance ratio between the drain electrode and the source electrode of the first transistor and the second transistor in which respective gate electrodes are biased by the first bias voltage power supply and the third transistor and the fourth transistor in which respective gate electrodes are biased by the second bias voltage power supply.
Herein, a third resistor and a fourth resistor can be made large within an allowable range of operation speed. Further, since the respective gate electrodes of the first transistor and the second transistor are biased to a fixed voltage, the capacity between the drain electrode and the source electrode is not dependent on the amplification rate of the first transistor and the second transistor. As a result, the level conversion circuit according to the present invention can be operated at a high speed and the size of the transistor can be reduced.